Through protein and DNA sequence analysis, much information on the primary structure of proteins has accumulated over the past several years. We have used this information to prepare antibodies reactive with defined portions of proteins. This is done by synthesizing 10-30 residue peptides that correspond to sections of the sequence of the protein. After coupling to a protein carrier, antisera to the peptides are prepared. In the case of transplantation and other lymphocyte antigens, these antibodies will be used to define sites of cellular recognition and serological specificities. In the case of the viral antigens the antisera will be used as specific probes to detect the expression of endogenous retroviral DNA found in human cells or as antisera specific for xenotrapic murine leukemia virus or murine mink cell focusing virus.